Self-propelled earth penetrators, or moles, typically comprise a reciprocating hammer or piston member which impacts on a forward anvil surface at one end of the mole for forward motion, then returns without impacting on a rear anvil surface and repeats this action to define a bore through which piping or cabling may be passed. After creating an underground bore, the mole is retracted by causing the hammer to impact on the rear anvil only and thus move it back out of the bore it has created.
Moles generally comprise a cylindrical member having a somewhat conical point for the forward boring movement. Within the cylindrical member, a hammer may reciprocate to impact on a forward anvil for operative boring and impact upon a rear anvil to retract the tool as previously stated. The direction of motion is controlled by various valving mechanisms which may be peculiar to each different mole structure.
Self-propelled earth penetrators or moles typically are driven by an internal linearly impacting hammer that is free to slide fore and aft. Between impacts of the hammer on a front anvil, the hammer must return to the rearmost possible position without impacting on the rear anvil. Accordingly, the hammer cycle must be completely controlled by the application and removal of fluid pressure at the proper times.
More specifically, after impact on the front anvil, at some midstroke point in the retract stroke while the hammer is being accelerated toward the rear anvil, the net force on the hammer must be switched to reverse direction. Thus, propulsion pressures are switched, the hammer decelerates to a stop, reverses its direction of motion, and then accelerates toward the forward anvil.
For forward penetration into the soil, the rear anvil must of course not be impacted; although it is desired that the maximum available stroke distance be utilized for hammer acceleration in the forward direction.
Thus the problem is one of switching the forces acting on the hammer at the proper time to make the hammer coast to a stop just before the rear anvil. At end-of-stroke, after the hammer has impacted the front anvil, the forces must again be switched to restore the initial pressure conditions and commence the next cycle.
There are no known commercially acceptable hydraulically operated moles. All known commercially used moles operate under pneumatic control. The pneumatic powered moles are prone to freezing in cold weather, and also require a pressure line lubricant to prevent undue frictional wear between the relatively movable parts of the device. Moreover, to reverse directions in the known pneumatically operated moles, only awkward methods have been utilized, such as by twisting the pressure line to actuate a valve within the mole.
Accordingly, a hydraulically operated mole would be preferable from the standpoint of a greater overall efficiency on a power in, power out, basis and arranged so that direction of impact of the hammer on the anvils, and therefore the direction of movement of the mole, may be controlled merely by reversing pressure between the input and output hydraulic lines. Such a hydraulically operated mole is disclosed in U.S. Pat. No. 3,642,076. However, the mole disclosed in this patent requires duplication of control valves at either end of the mole to determine the direction of travel.
The present invention provides a new and improved hydraulically operated mole in which the direction of travel may be controlled dependent on which line hydraulic pressure is applied to and which includes a valving mechanism of simplified construction which controls the propulsion and retraction of the hammer in either direction of operation.